In a case where an AC electric motor is used as a drive source and where the AC electric motor is controlled by an inverter, regenerative energy cannot be consumed, and a DC voltage rises, if a load which consumes the regenerative energy does not exist on a power source side (that is, an overhead line side) in a regeneration mode.
Especially in case of an electric train, usually the electric train being braked converts the inertial energy of a vehicle into electric energy and restores the regenerative energy to an overhead line (that is, a power source).
In this case, it is indispensable that the power source side has a regenerativity, and that another electric train being accelerated as acts as a regenerative load exists on the power source side.
Here, in a case where the regenerative energy of the electric train in the regeneration mode is greater than energy which is consumed by the electric train being accelerated, the voltage of the overhead line or the voltage of a filter capacitor disposed at a stage preceding the inverter increases, and a protective function is sometimes actuated by an overvoltage.
A method for improving this problem is ordinarily such that the regenerative energy is fined to lower a braking force based on an electric brake, and that a lowered component is compensated by a mechanical brake.
In this case, a brake shoe wears off due to the operation of the mechanical brake, and hence, the maintenance of the brake shoe needs to be performed in a certain fixed period.
Besides, in JP-A-2002-95299 (Patent Document 1), there is stated a drive control apparatus for an electric train, having an inverter which receives feed of electric power through an overhead line and which regenerates electric power through the overhead line during a brake operation, and an induction motor which is driven by the inverter; comprising detection means for detecting a load state of an overhead line side, and current proportion adjustment means for adjusting a proportion of a magnitude of a torque current to a magnitude of an excitation current of the induction motor so that a loss in a secondary resistance of the induction motor may increase, in a case where a load of the overhead line side during a regenerative operation is a light load.    Patent Document 1: JP-A-2002-95299